It depends, one could be very lopsided, if one's training was deliberately uneven, for me my strength is about the same, I think a 3:2 ratio of triceps to biceps strength is standard... I could be wrong though.

Why do you want to know? You should aim to improve each, whichever is naturally stronger over time and with work for you is probably so for most people...

You should be fine to start with more, sorry I misunderstood. BTW, you will know if the weight is too light by some minor pain in your elbows. Don't mistakenly see this as using too much weight. It's kinda counter-intuitive, but using the right weight for skullcrushers feels best anyway.

Yeah sure, I usually barbell/preacher curl 65, and I do 85 for skullcrushers, you should start with at least 85 I guess. Well, then again, if you're just beginning work on the triceps you might want to start lower. Whatever you can do three sets of eight with is best.